End-tidal gas analysis apparatus for respirators

ABSTRACT

Intermittently operated gas analysis apparatus and a method for monitoring the proportion of particular gases carried in the endtidal portion of breathing. An analysis sample is obtained, following a predetermined time delay, when a patient expires air into a respirator. The apparatus is rapidly deactuated at the beginning of inspiration, and remains off until the patient again expires breath. By an appropriate setting of the time delay, a gas sample that accurately represents the end-tidal condition may be obtained, making the analyzer particularly suitable for monitoring the CO2 content of expired breath.

United States Patent 1191 1111 3,910,261 Ragsdale et al. Oct. 7, 1975 [5 END-TIDAL GAS ANALYSIS APPARATUS 1,915,959 10/1970 Germany 128/2 c FOR RESPIRATORS OTHER PUBLICATIONS [75] Inventors: Charles Ragsdale Riverside; Auto. Measurement of Lung Function The Lancet,

James Weigl, Rialto, bOth Of Calif. Sept 1 1965 pp 573 574 [73] Assignee: Bourns, Inc., Riverside, Calif.

Primary Examiner-Richard A. Gaudet [22] Flled: June 1974 Assistant ExaminerLee S. Cohen [21] Appl. No.: 478,429 Attorney, Agent, or FirmRichard S. Koppel; William G. Becker [52] US. Cl l28/2.07; 73/421.5 R; 23/254 R 51 1111.01. A61B 5/00 [57] ABSTRACT [58] Field of Search 128/2.07, 2.08, 2 R, 2 C; lntermittently operated gas analysis apparatus and a 73/4215, 23; 23/254 C, 254 R method for monitoring the proportion of particular gases carried in the end-tidal portion of breathing. An

[56] References Cited analysis sample is obtained, following a predetermined UNITED STATES PATENTS time delay, when a patient expires air into a respirator.

3,306,283 2/1967 Arp 128/2.07 P F is rapidly. deactuateid at the .beginnirlg 3395,70l 8/1968 Bartlett ct alflm 128/2 08 of inspiration, and remains off until the patient again 3,401,683 9/1968 Webb et al. 1 128/2.07 expires breathy an appropriate Setting of the time 3, 13, 5 10 1971 Gorsuch 123/107 X delay, a gas sample that accurately represents the end- 3,661,528 5/1972 Falk 73/4215 tidal condition may be obtained, making the analyzer FOREIGN PATENTS OR APPLICATIONS particularly suitable for monitoring the CO content of expired breath. 829,409 3/1960 United Kingdom 128/2.07 261,798 5/1968 Austria 128/2.07 9 Claims, 4 Drawing Figures p v 1 6A5 155; g Y 30 ANALYZER PROCESS/N6 CIRCUIT I 26 1 I 5UPPL Y EXP/RA TORY SEA/50R U.S. Patent Oct. 7,1975 Sheet 2 of 2 3,910,261

3/1/2167 4 dW/7d END-TIDAL GAS ANALYSIS APPARATUS FOR RESPIRATORS BACKGROUND OF THE INVENTION This inention relates to a gas analysis method and apparatus, and more particularly to a method and apparatus for analyzing the end-tidal portion of gas expired by a patient into a respirator. I

It is often desirable to analyze the expired breath of a respirator patient to determine, for example, the arterial CO content. This measurement is complicated by the nonuniformity of the CO concentration, which begins at a small amount and gradually builds up until a plateau is reached. The CO level at the plateau, which is commonly referred to as the end-tidal region, is the true indicator of arterial condition. Gas samples obtained at other times, either prior to the beginning of the end-tidal region or after expiration has ended and the patient is inhaling, can produce an artificially low CO reading.

One popular gas analysis system employs a sensor in the expiratory path of a respirator to measure the air flow issuing from a patient and control the operation of the analysis mechanism in response thereto. The sensor detects the beginning of expiration, which is character ized by a large outflow of air, and produces a signal to begin sampling the expired gas. A timer is provided to delay the sampling long enough for the end-tidal region to be reached, after which sampling takes place for as long as the expiratory air flow remains strong enough to sustain a signal from the sensor. Sampling is terminated when the air flow falls below this level, and the sampling system is reset for the next breath cycle.

While this system generally restricts sampling to the expiratory phase of breathing, a problem exists in that the expiratory air flow may fall below the threshold rate for terminating sampling near or even before the beginning of the end-tidal region. If sampling does terminate before the end-tidal region is reached, any CO reading will not accurately reflect the arterial condition. Even if sampling terminates after the end-tidal region has begun, the possibility of premature sampling may encourage the choice of time delay for beginning sampling that runs out considerably before the end-tidal region, again producing an erroneous arterial CO indication. In any event, the very small expiratory flow which characterizes the latter portion of the end-tidal region has rendered attempts to sample for the full end-tidal period quite difficult.

In another system known to the art gas samples are obtained at a location comparatively remote from the patient, downstream from the valve which shuts off the expiratory conduit during inspiration, and sampling is not begun until the expiratory phase has terminated and inspiration has been detected. While this system endeavors to eliminate the problems associated with sampling before the end-tidal region has been reached, the advantages of obtaining gas samples directly as they are expired from the patient are lost.

SUMMARY OF THE INVENTION In view of the above-stated problems existing in the prior art, it is an object of the present invention to provide a novel and improved system and method for analyzing the end-tidal portion of expired breath.

Another object is the provision of a novel and improved system and method by which gas samples are obtained only during the end-tidal portion of expiration, and which permit samples to be taken over a substantial amount of that portion.

Still another object is the provision of novel and improved apparatus and a method for analyzing the endtidal portion of expiration in which the acquisition of gas samples is controlled in response to large gas flows.

In the accomplishment of these and other objects, an end-tidal gas analysis apparatus and method is provided by the present invention for use with a respirator of the type having an air source actuated in response to patient demand, and a conduit system including inspiratory and expiratory conduits for respectively delivering air to and receiving air from the patient. Broadly described, the apparatus includes a gas analyzing device capable of testing gas within a respirator conduit system, an actuator adapted to initiate operation of the analyzing device a predetermined period of time following the beginning of expiration, and a deactuator which senses the beginning of patient inspiration and rapidly terminates operation of the analyzing device in response thereto. With an appropriate interval between the beginning of expiration and analysis, the analysis period may be set to occur substantially entirely within the end-tidal period, and to extend over a large portion thereof.

According to more particular features of the invention, gas flow rate sensors are disposed in both the inspiratory and expiratory conduits and deliver input signals to a signal processing circuit when gas flows are detected. The processing circuit includes an amplifier, a threshold device adapted when its threshold is exceeded to terminate operation of the analyzing device, A.C. coupling between the amplifier and threshold device, and a DC. restoration circuit between the A.C. coupling and the threshold device. DC. bias is substantially removed by the restoration circuit from the signal applied to the threshold device, thereby enabling the threshold level to be rapidly exceeded and analysis terminated when inspiration begins.

According to another feature, a means such as a pump is provided to extract gas samples from the respirator conduit system for delivery to the analysis device. By sampling between flow control valves in the inspiratory and expiratory conduits, expired gas may be obtained directly from the patient while expiration is still going on.

Further objects, features, and advantages of the invention will become apparent to those skilled in the art from the following detailed description of a particular embodiment, along with the drawings in which:

FIG. 1 is a somewhat idealized graph of a respirator patients breathing cycle, with the CO concentration in the respirator conduit system indicated in dashed lines;

FIG. 2 is a functional diagram of the analysis apparatus shown in conjunction with a respirator system;

FIG. 3 is a block diagram of electrical circuitry employed in the invention; and

FIG. 4 is a schematic diagram of a processing circuit for controlling the operation of the gas analysis apparatus in response to patient inspiration and expiration.

DETAILED DESCRIPTION Referring first to FIG. 1, a typical air-flow pattern to and from a patient assisted by a respirator is shown, with inspiration indicated by a negative flow level and expiration by a positive flow level. At the end of inspiration (coinciding with the beginning of expiration),

designated time T in the figure, a large flow of air is expelled by the patient into the respirator. The outward air flow gradually tapers off to a very low level, indicated by reference numeral 2, followed by a pause with essentially no air flow until the patient again draws breath at time T Thereafter the breathing sequence continues in a repetitive sequence of inspirations and expirations.

An indication of the patients arterial CO concentration may be obtained by analyzing his expired breath; this in turn can be accomplished by measuring the CO level in the respirator conduit or air tube network. This latter quantity is indicated by a dashed line in FIG. 1. The CO concentration during inspiration, indicated by reference numeral 4, is generally that of the ambient atmosphere, or about 0.03%. During expiration the concentration climbs from the ambient level to a plateau 6 at a peak of about 5.4% for healthy patients. The plateau 6, which extends through approximately the final third of expiration, is referred to as the end-tidal level and is the true indicator of arterial CO concentration. It will be noted from the drawing, however, that the end-tidal region may be reached near or even after the time the expiration flow has fallen to a low level 2. If this flow rate is used to indicate the end of expiration, at which time sampling is terminated, the samples obtained may represent a period prior to the end-tidal region and an artificially low CO reading will result.

Briefly stated, the present invention achieves an accurate end-tidal sample by sensing the beginning of pa tient expiration at T initiating sampling during the end-tidal region at a time T after expiration begins, and rapidly terminating sampling when the next inspiration is sensed at time T Sampling may be conducted during a large portion of the end-tidal region, and con trol functions need be operated only in response to large air flows.

A respirator system employing gas analysis apparatus that achieves this result is shown in block diagram form in FIG. 2.'The respirator includes an air supply device 8, which provides a desired air mix and pumps the air through a check valve 10 into an inspiratory conduit such as plastic tube 12 in response to patient demand. Expired air is routed through an expiratory tube 14 to a chamber 16, and thence out of the respirator system through a continuation expiratory tube 18. Air is channeled to and from the patient through a tap 20, while an inflatable bladder valve 22 is alternately expanded to close off the exit from expiratory tube 14 during inspiration, and relaxed during expiration to permit the discharge of expired air. Thus far, the described system is well known to the art, and the construction details thereof may be tailored in any of several convenient ways according to the needs of the user.

The novel gas analysis apparatus includes an air pump 24 that is tapped by a tube 26 into the conduit system at a location between valves 10 and 22 where expired air can be extracted, a gas analysis device 28 such as a thermal conductivity cell adapted to receive air from pump 24, and an electronic processing circuit 30 controlling the operation of pump 24. A first air flow sensor such as termistor 32 is inserted into continuation expiratory tube 18 to produce a signal for delivery to processing circuit 30 when an expiratory air flow is detected. A second sensor such as thermistor 34 is inserted into the inspiratory tube 12 to sense the beginning of patient inspiration and in response deliver a second signal to the processing circuit 30. The triggering flow rate for each of the thermistors 32 and 34 is generally from one to two liters per minute, although the trigger levels may be varied as desired. For respirators which incorporate an inspiratory flow sensor to control a breath assist mechanism, this sensor may also be used in the analysis apparatus instead of thermistor 34.

Referring now to FIG. 3 for a block diagram of processing circuit 30, a bias circuit 36 supplies a voltage sufficient to establish a heating current through the thermistors comprising expiratory sensor 32 and inspiratory sensor 34. The expiratory sensor 32 is connected via an A.C. coupling circuit 38 to a threshold comparator 40, where the signal is compared with a reference threshold signal and an output produced for delivery to the set input of a flip-flop 42 when the threshold is ex ceeded. The output of the flip-flop is in turn connected to the drive mechanism for pump 24.

Inspiratory sensor 34, also heated by bias circuit 36, is connected to a D.C. amplifier 44, having a gain factor of 8.5, and thence to an A.C. coupling and D.C. restoration circuit 46, where the A.C. component of the amplified sensor signal undergoes a D.C. shift. The output of circuit 46 is applied to a threshold device such as comparator 48, which is turn has an output con nected to reset the flip-flop 42. The aforesaid D.C. shift is such that threshold device 48 is rapidly triggered at the beginning of inspiration to produce an output which resets the flip-flop 42.

A delay circuit 49 receives the set output of flip-flop 42 and, after a predetermined delay period, emits a pulse to energize the drive mechanism for pump 24. The delay is set at about two-thirds of the expiratory period, or generally in the area of 7.5 seconds. The pump drive energizing signal is rapidly terminated, without appreciable delay, when flip-flop 42 is reset.

Circuit details are shown in FIG. 4 of the processing circuit block diagram functions illustrated in FIG. 3. A bias voltage of -IS volts is applied to an input terminal 50, which is connected to a resistor 52. The expiratory an inspiratory thermistors 32 and 34 are connected between ground and circuit nodes 53 and 54, respectively, and thereafter through resistors 55 and 56 to the other side of resistor 52 from terminal 50. The thermistors and their associated resistors are paralleled by a zenor diode 57 and normally carry a current of about 5.3 ma for still air. The expiratory thermistor 32 is also A.C. coupled to an operational amplifier which serves as comparator 40 through an RC circuit comprising a series connected resistor 58 and capacitor 60 and a further resistor 62 between ground and capacitor 60, the thermistor output being applied at the signal input 64 to comparator 40. The reference input 66 of comparator 40 receives a reference signal from a voltage divider circuit which comprises resistors 68 and 70 connected between input terminal 50 and ground. The output of comparator 40 is connected via appropriate coupling circuitry to the set input of flip-flop 42, which consists lines, the output 76 of which leads to A.C. coupling and DC. restoration circuit 46, also shown enclosed in dashed lines. This latter circuit is formed from a series connected resistor 78 and capacitor 80, together with a diode 82 which provides a conductive path to ground on the far side of the series circuit from amplifier 44. The circuit output is connected to the signal input 84 of comparator 48, the reference input 86 of which is supplied with a reference voltage from a voltage divider circuit which comprises resistors 88 and 90 connected between input terminal 50 and ground. The output of comparator 48 is connected via appropriate coupling to the other input of flip-flop 42.

A monostable multivibrator 92 located at the output of flip-flop 42 introduces a delay to the output signal, the duration of the delay being selectively adjustable by means of an'adjustment mechanism 94. The output of flip-flop 42 and the multivibrator 92 are fed to an AND gate 96, the output of which is applied to the drive mechanism for air pump 24.

In operation, the respirator functions in the usual manner with inspiratory air flows supplied on patient demand, followed by an expiration period. Immediately before expiration begins, there is little or no air flow past expiratory thermistor 32, and its resistance is relatively low. A steady state voltage of approximately-1.5 volts is thereby established at node 53. The air flow at the beginning of expiration cools thermistor 32, increasing its resistance so that a negative pulse of from about 0.2 to 1.0 volts for the average person is superimposed onto the steady state voltage at node 53. This pulse triggers comparator 40 to set flip-flop 42, delivering an input to multivibrator 92. After the predetermined delay period the multivibrator produces an output which, together with the continuing output from flip-flop 42, triggers AND gate 96. This actuates the pump 24, which now begins to extract air from the respirator tube network for delivery to the analyzer 28. i

The extraction of a gas sample from the respirator tubes continues until the beginning of inspiration, at which time the inspiratory air flow cools the thermistor 34, increasing its resistance so that a negative voltage pulse is produced at node 54. Absolute voltage levels at node 54 are approximately the same as at node 53 or a 1.5 volt D.C. level with a 0.2 to 1.0 volt A.C. component. The inspiratory signal is amplified by amplifier 44, then A.C. -coupled by capacitor 80 to the comparator 48. Without diode 82, the A.C. coupled signal from capacitor 80 would be shifted in the positive direction, introducing a delay before the signal level could fall to a level low enough to trigger the comparator 48. Diode 82 avoids this problem by becoming conductive during the intervals between inspiration signals, holding the voltage at its anode to about 0.7 volts and charging the capacitor 80. At the appearance of an inspiration pulse, the threshold level of comparator 48 is rapidly exceeded (in a negative direction) so that it produces an output that resets flip-flop 42, thereby removing one of the inputs to AND gate 96 and very rapidly deactuating pump 24.

Intermittent pumping continues in this fashion during successive breathing cycles, with the pump 24 operating only during the end-tidal period. End-tidal gas is accumulated in analysis device 28, and an accurate indication of the arterial CO level or other end-tidal char acteristic may be obtained.

While a particular embodiment of the invention has been shown and described, numerous additional modifications and variations are possible in light of the above teachings. It is therefore intended that the scope of the invention be limited only in and by the terms of the appended claims.

What is claimed is:

1. End-tidal gas analysis apparatus suitable for use with a respirator of the type having an air source actuated in response to patient demand and a conduit system which includes inspiratory and expiratory conduits for respectively delivering air to and receiving air from a patient, said apparatus comprising:

means for analyzing gas contained in said conduit system,

means for supplying gas from said conduit system to said analyzing means, actuator means adapted to sense the beginning of patient expiration and to initiate operation of said gas supplying means after a predetermined delay period following the beginning of said patient expiration, and deactuator means adapted to sense the beginning of patient inspiration-and rapidly terminate operation of said gas supplying means in response thereto,

said actuator means and deactuator means cooperating to operate said gas supplying means for substantially the full interval between the end of said delay period and the beginning of patient inspiration, and to inhibit operation of said gas supplying means during substantially the full remainder of the time the analysis apparatus is in use.

2. Apparatus according to claim 1, wherein said deactuator means includes a flow rate sensor adapted to sense gas flows associated with the beginning of inspiration.

3. Apparatus according to claim 1, wherein said gas supplying means includes means for extracting a gas sample from said respirator conduit system for delivery to said analyzing means, the operation of said extracting means being controlled by said actuator means and deactuator means.

4. Apparatus according to claim 1, wherein said deactuator means includes a gas flow rate sensor disposable in the inspiratory conduit, said sensor being adapted to produce a signal in response to a gas flow through said inspiratory conduit exceeding a predetermined threshold level, and a processing circuit means for a signal produced by said sensor.

5. End-tidal gas analysis apparatus suitable for use with a respirator of the type having an air source actuated in response to patient demand, a conduit system which includes inspiratory and expiratory conduits for respectively delivering air to and receiving air from a patient, and gas flow control valves disposed respectively in said inspiratory and expiratory conduits, said apparatus comprising:

means for analyzing gas contained in said conduit system,

means for supplying gas from the portion of said conduit system between said flow control valves to said analyzing means,

actuator means adapted to sense the beginning of patient expiration and to initiate operation of said gas suppling means after a predetermined period of time following the beginning of said patient expiration, and

deactuator means adapted to sense the beginning of patient inspiration and rapidly terminate operation of said gas supplying means in response thereto,

said actuator means and deactuator means cooperat ing to operate said gas supplying means for substantially the full interval between the end of said delay period and the beginning of patient inspiration, and to inhibit operation of said gas supplying means during substantially the full remainder of the time the analysis apparatus is in use.

6. End-tidal gas analysis apparatus suitable for use with a respirator of the type having an air source actuated in response to patient demand and a conduit system which includes inspiratory and expiratory conduits for respectively delivering air to and receiving air from a patient, said apparatus comprising:

a gas analysis device;

a pump means for pumping gas from said conduit system to said analysis device;

a signal processing circuit means controlling the operation of said pump means; and

first and second gas flow sensors disposable respectively in said expiratory and inspiratory conduits, said first and second sensors being respectively connected to said signal processing circuit means to provide input signals when the sensed gas flows exceed predetermined threshold rates;

said signal processing circuit means including means to actuate said pump means after a predetermined period of time following receipt of an input signal from said first sensor, and further including rapid response means to deactuate said pump means in response to an input signal from said second sensor.

7. End-tidal gas analysis apparatus suitable for use with the air supply of a respirator, comprising:

a conduit system including inspiratory and expiratory conduits for respectively delivering air from said air supply to and receiving air from a patient,

gas flow control valves disposed respectively in said inspiratory and expiratory conduits,

means for analyzing gas contained in said conduit system,

means for supplying gas from said conduit system to said analyzing means, including means for extracting a gas sample from the portion of said conduit system between said flow control valves,

actuator means adapted to sense the beginning of patient expiration and to initiate operation of said extracting means after a predetermined delay period following the beginning of said patient expiration, and

deactuator means adapted to sense the beginning of patient inspiration and rapidly terminate operation of said analyzing means in response thereto.

8. The method of analyzing end-tidal gas expired by a patient into a respirator conduit system, comprising the steps of:

providing a gas analysing means and a means for supplying gas thereto from said conduit system; sensing an expiratory gas flow into the conduit system from the patient;

actuating the gas supplying means after a predetermined period of time following the beginning of expiration sensed in the conduit systems:

sensing an inspiratory gas flow from the conduit system to the patient; and

rapidly deactuating the gas supplying means in response to the beginning of inspiratory gas flow sensed in the conduit system 9. The method of claim 8, suitable for measuring the end-tidal CO content of an expiratory breath, wherein said predetermined time period is at least sufficient to reach the end tidal portion of expiration. 

1. End-tidal gas analysis apparatus suitable for use with a respirator of the type having an air source actuated in response to patient demand and a conduit system which includes inspiratory and expiratory conduits for respectively delivering air to and receiving air from a patient, said apparatus comprising: means for analyzing gas contained in said conduit system, means for supplying gas from said conduit system to said analyzing means, actuator means adapted to sense the beginning of patient expiration and to initiate operation of said gas supplying means after a predetermined delay period following the beginning of said patient expiration, and deactuator means adapted to sense the beginning of patient inspiration and rapidly terminate operation of said gas supplying means in response thereto, said actuator means and deactuator means cooperating to operate said gas supplying means for substantially the full interval between the end of said delay period and the beginning of patient inspiration, and to inhibit operation of said gas supplying means during substantially the full remainder of the time the analysis apparatus is in use.
 2. Apparatus according to claim 1, wherein said deactuator means includes a flow rate sensor adapted to sense gas flows associated with the beginning of inspiration.
 3. Apparatus according to claim 1, wherein said gas supplying means includes means for extracting a gas sample from said respirator conduit system for delivery to said analyzing means, the operation of said extracting means being controlled by said actuator means and deactuator means.
 4. Apparatus according to claim 1, wherein said deactuator means includes a gas flow rate sensor disposable in the inspiratory conduit, said sensor being adapted to produce a signal in response to a gas flow through said inspiratory conduit exceeding a predetermined threshold level, and a processing circuit means for a signal produced by said sensor.
 5. End-tidal gas analysis apparatus suitable for use with a respirator of the type having an air source actuated in response to patient demand, a conduit system which includes inspiratory and expiratory conduits for respectively delivering air to and receiving air from a patient, and gas flow control valves disposed respectively in said inspiratory and expiratory conduits, said apparatus comprising: means for analyzing gas contained in said conduit system, means for supplying gas from the portion of said conduit system between said flow control valves to said analyzing means, actuator means adapted to sense the beginning of patient expiration and to initiate operation of said gas suppling means after a predetermined period of time following the beginning of said patient expiration, and deactuator means adapted to sense the beginning of patient inspiration and rapidly terminate operation of said gas supplying means in response thereto, said actuator means and deactuator means cooperating to operate said gas supplying means for substantially the full interval between the end of said delay period and the beginning of patient inspiration, and to inhibit operation of said gas supplying means during substantially the full remainder of the time the analysis apparatus is in use.
 6. End-tidal gas analysis apparatus suitable for use with a respirator of the type having an air source actuated in response to patient demand and a conduit system which includes inspiratory and expiratory conduits for respectively delivering air to and receiving air from a patient, said apparatus comprising: a gas analysis device; a pump means for pumping gas from said conduit system to said analysis device; a signal processing circuit means controlling the operation of said pump means; and first and second gas flow sensors disposable respectively in said expiratory and inspiratory conduits, said first and second sensors being respectively connected to said signal processing circuit means to provide input signals when the sensed gas flows exceed predetermined threshold rates; said signal processing circuit means including means to actuate said pump means after a predetermined period of time following receipt of an input signal from said first sensor, and further including rapid response means to deactuate said pump means in response to an input signal from said second sensor.
 7. End-tidal gas analysis apparatus suitable for use with the air supply of a respirator, comprising: a conduit system including inspiratory and expiratory conduits for respectively delivering air from said air supply to and receiving air from a patient, gAs flow control valves disposed respectively in said inspiratory and expiratory conduits, means for analyzing gas contained in said conduit system, means for supplying gas from said conduit system to said analyzing means, including means for extracting a gas sample from the portion of said conduit system between said flow control valves, actuator means adapted to sense the beginning of patient expiration and to initiate operation of said extracting means after a predetermined delay period following the beginning of said patient expiration, and deactuator means adapted to sense the beginning of patient inspiration and rapidly terminate operation of said analyzing means in response thereto.
 8. The method of analyzing end-tidal gas expired by a patient into a respirator conduit system, comprising the steps of: providing a gas analysing means and a means for supplying gas thereto from said conduit system; sensing an expiratory gas flow into the conduit system from the patient; actuating the gas supplying means after a predetermined period of time following the beginning of expiration sensed in the conduit systems: sensing an inspiratory gas flow from the conduit system to the patient; and rapidly deactuating the gas supplying means in response to the beginning of inspiratory gas flow sensed in the conduit system
 9. The method of claim 8, suitable for measuring the end-tidal CO2 content of an expiratory breath, wherein said predetermined time period is at least sufficient to reach the end tidal portion of expiration. 